asterisk users - Apr 2003 - SIP transfer doesn't stop music on hold

Has anyone encountered this SIP problem? I dial a SIP phone from a ZAP phone
and after talking for a few minutes the SIP user transfers the ZAP user to
another ZAP user. The transfer works correctly, but when the native bridge
occurs

    -- Attempting native bridge of Zap/22-1 and Zap/16-1

the music on hold never stops. The two lines are bridged correctly and you
can hear one another but you essentially have to speak OVER the music on
hold.

When I do the same thing using three ZAP phones

    -- Attempting native bridge of Zap/22-2 and Zap/16-1
    -- Stopped music on hold on Zap/20-1

The music on hold stops correctly. Does anyone know if this is

 - a config defect on my part?
 - some quirk with my SIP phone (BudgetTone 102D)?
 - an asterisk bug?

If anyone has a similar setup and can reproduce this test I'd like to know
how it goes.

Thanks